1. Field of the Invention
The present invention relates to a bandgap voltage reference circuit and, more particularly, to a bandgap voltage reference circuit with an increased difference voltage .DELTA.V.sub.BE.
2. Description of the Related Art
A bandgap voltage reference circuit is a circuit that provides a reference voltage that is ideally temperature independent. Bandgap voltage reference circuits are commonly used as stand-alone voltage sources, and as building blocks in analog-to-digital converters, digital-to-analog converters, bias line generators, and other common analog circuits.
FIG. 1 shows a schematic diagram that illustrates a conventional bandgap voltage reference circuit 100. As shown in FIG. 1, circuit 100 includes a current source 110 that outputs a current I that is proportional to absolute temperature (PTAT), and transistors Q1, Q2, and Q3. The collectors of transistors Q1 and Q2 are connected to current source 110 through resistors R1 and R2, respectively, while the collector of transistor Q3 is directly connected to current source 110.
In addition, the emitters of transistors Q1 and Q3 are connected together, while the emitter of transistor Q2, which has an emitter area that is N times larger than the emitter area of transistor Q1, is connected to the emitter of transistor Q1 through resistor R3. Further, the bases of transistors Q1 and Q2 are connected to the collector of transistor Q1, while the base of transistor Q3 is connected to the collector of transistor Q2.
In operation, circuit 100 provides a nearly temperature independent reference voltage V.sub.REF between the collector and emitter of transistor Q3 by summing a voltage that has a positive temperature coefficient with voltage that has a negative temperature coefficient of equal value.
For example, when the temperature increases by one degree, the voltage with the positive temperature coefficient increases by, for example, 2 mV while the voltage with the negative temperature coefficient decreases by 2 mV. Since the voltages vary an equal amount in opposite directions, the reference voltage V.sub.REF remains unchanged when the temperature increases by one degree.
With respect to the voltage with the positive temperature coefficient, it is known that the difference between the base-to-emitter voltages of a pair of bipolar transistors that are forced to operate with unequal emitter current densities is a voltage with a positive temperature coefficient.
In circuit 100, since transistor Q2 has an emitter area that is N times larger than the emitter area of transistor Q1, transistors Q1 and Q2 operate with unequal emitter current densities. As a result, a difference voltage .DELTA.V.sub.BE, which is equal to V.sub.BEQ1 -V.sub.BEQ2, has a positive temperature coefficient.
As shown in FIG. 1, the base-to-emitter voltage V.sub.BEQ1 of transistor Q1 is equal to the base-to-emitter voltage V.sub.BEQ2 of transistor Q2 and the voltage VR3 across resistor R3, i.e., V.sub.BEQ1 =V.sub.BEQ2 +VR3. Rearranging yields V.sub.BEQ1 -V.sub.BEQ2 =VR3.
Since the difference voltage .DELTA.V.sub.BE is equal to the difference between the base-to-emitter voltages (.DELTA.V.sub.BE =V.sub.BEQ1 -V.sub.BEQ2), the difference voltage .DELTA.V.sub.BE is also equal to the voltage VR3 across resistor R3. Since the difference voltage .DELTA.V.sub.BE has a positive temperature coefficient, the voltage VR3 across resistor R3 must also have a positive temperature coefficient.
The voltage VR3 across resistor R3 (and the value of resistor R3) define the resistor current which, in turn, defines the emitter current I.sub.EQ2 of transistor Q2. As a result, the emitter current I.sub.EQ2 is proportional to the difference voltage .DELTA.V.sub.BE and, therefore, must have a positive temperature coefficient.
In addition, the collector current I.sub.CQ2 of transistor Q2 is approximately equal to the emitter current I.sub.EQ2 of transistor Q2 due to the beta of transistor Q2. As a result, the collector current I.sub.CQ2 of transistor Q2 is proportional to the difference voltage .DELTA.V.sub.BE and, therefore, must have a positive temperature coefficient.
Thus, since the collector current I.sub.CQ2 is proportional to the difference voltage .DELTA.V.sub.BE, the voltage VR2 across resistor R2 is proportional to the difference voltage .DELTA.V.sub.BE, and therefore must also have a positive temperature coefficient.
The voltage VR2 is also known as an amplified difference voltage .DELTA.V.sub.BE because the voltage VR2 is approximately equal to R2/R3 times the voltage VR3 which, in turn, is equal to the difference voltage .DELTA.V.sub.BE.
With respect to the voltage with the negative temperature coefficient, it is known that the base-to-emitter voltage of a bipolar transistor has a negative temperature coefficient when the collector current of the transistor is proportional to absolute temperature.
As noted above, current source 110 outputs a current I that is proportional to absolute temperature. As a result, the base-to-emitter voltage V.sub.BEQ3 of transistor Q3 has a negative temperature coefficient.
Thus, circuit 100 provides a nearly temperature independent reference voltage V.sub.REF between the collector and emitter of transistor Q3 by summing the voltage VR2, the amplified difference voltage .DELTA.AV.sub.BE, with the base-to-emitter voltage V.sub.BEQ3 across the base-to-emitter junction of transistor Q3.
The amplified difference voltage .DELTA.AV.sub.BE (VR2) has a positive temperature coefficient of approximately +2 mV/.degree. C., while the base-to-emitter voltage V.sub.BEQ3 has a negative temperature coefficient of approximately -2 mV/.degree. C. Thus, by summing voltages which have equal and opposite temperature coefficients, the total voltage, i.e., the reference voltage V.sub.REF, remains unchanged as the temperature changes. (See also U.S. Pat. No. 3,617,859 to Dobkin which is hereby incorporated by reference.)
FIG. 2 shows a schematic diagram that illustrates a conventional bandgap voltage reference circuit 200. Circuit 200 is similar to circuit 100 and, as a result, utilizes the reference numerals to designate the structures which are common to both circuits.
As shown in FIG. 2, circuit 200 differs from circuit 100 in that circuit 200 eliminates both current source 110 and transistor Q3, and instead utilizes an operational amplifier (op amp) 210 and a resistor R4. As with circuit 100, transistor Q2 of circuit 200 has an emitter area that is N times larger than the emitter area of transistor Q1 of circuit 200.
Op amp 210 has a positive input connected to the collector of transistor Q1, a negative input connected to the collector of transistor Q2, and an output connected to the bases of transistors Q1 and Q2. Resistor R4, in turn, has a first end connected to resistor R3 and the emitter of transistor Q1, and a second end connected to ground.
In operation, the resistances of resistors R1 and R2 are equal, and develop voltages at the collectors of transistors Q1 and Q2 which are equal when the collector currents are equal. When the collector currents, which are proportional to absolute temperature, are not equal, op amp 210 responds to the unequal collector voltages by changing the base voltages of transistors Q1 and Q2 until the collector currents of transistors Q1 and Q2 are equal.
In circuit 200, transistors Q1 and Q2 are again forced to operate with unequal emitter current densities due to the difference in emitter areas. As a result, the difference voltage .DELTA.V.sub.BE is again equal to the voltage VR3 across resistor R3, and the voltage VR3 again has a positive temperature coefficient.
The voltage VR3 across resistor R3 defines the emitter current I.sub.EQ2 of transistor Q2. As a result, the emitter current I.sub.EQ2 is proportional to the difference voltage .DELTA.V.sub.BE, and must have a positive temperature coefficient.
Since the collector currents, the base currents, and the betas of transistors Q1 and Q2 are nominally the same, the emitter current I.sub.EQ1 of transistor Q1 is nominally the same as the emitter current I.sub.EQ2 of transistor Q2. Thus, the emitter current I.sub.EQ1 of transistor Q1 is also proportional to the difference voltage .DELTA.V.sub.BE.
Since both the emitter current I.sub.EQ1 of transistor Q1 and the emitter current I.sub.EQ2 of transistor Q2 are proportional to the difference voltage .DELTA.V.sub.BE, the combined currents through resistor R4 must also be proportional to the difference voltage .DELTA.V.sub.BE, and must also have a positive temperature coefficient.
Since the combined emitter currents have a positive temperature coefficient, the voltage VR4 across resistor R4 must also have a positive temperature coefficient. Thus, by properly sizing resistor R4 to obtain the proper gain, the amplified difference voltage .DELTA.AV.sub.BE is defined across resistor R4.
In circuit 200, the amplified difference voltage .DELTA.AV.sub.BE (the voltage VR4) is summed with the base-to-emitter voltage V.sub.BEQ1 of transistor Q1 to produce the reference voltage V.sub.REF. The base-to-emitter voltage V.sub.BEQ1 of transistor Q1 has a negative temperature coefficient as op amp 210 insures that transistor Q1 receives a collector current that is proportional to absolute temperature. (See also U.S. Pat. No. 3,887,863 to Browkaw which is hereby incorporated by reference.)
Although circuits 100 and 200 output reference voltages V.sub.REF which are, to a first degree, constant over variations in temperature, in actual practice the reference voltages V.sub.REF vary slightly with changes in temperature. Thus, with the need to produce highly-accurate, low-voltage reference voltages, there is a need for a bandgap voltage reference circuit that reduces these slight changes in the reference voltage V.sub.REF over changes in temperature.